A={x|-1≤X≤a,a＞-1},B={y|y=X+1,x∈A｝=[0,a+1],C=y|y=X²，x∈A｝={[a^2,1](-1{[0,1](0<=a<=1),{[0,a^2](a>1).由B=C得a+1=1(0<=a<=1),或a+1=a^2(a>1),解得a=0,或a=(1+√5）/2.
因为B=C所以 y=X+1=X²， x∈A 解得 x=(1+√3)/2 或x=(1-√3)/2 又 x∈A则a≧(1+√3)/2
